2.1.8 Path

2.1.8 Path

This line shows the path the article took to
reach the current system. When a system forwards the
message, it should add its own name to the list of systems
in the Path line. The names may be separated by any
punctuation character or characters, thus
"cbosgd!mhuxj!mhuxt", "cbosgd, mhuxj, mhuxt", and
"@cbosgd.uucp,@mhuxj.uucp,@mhuxt.uucp" and even
"teklabs, zehntel, sri-unix@cca!decvax" are valid
entries. (The latter path indicates a message that passed
through decvax, cca, sri-unix, zehntel, and teklabs, in
that order.) Additional names should be added from the
left, for example, the most recently added name in the
third example was "teklabs". Letters, digits, periods
and hyphens are considered part of site names; other
punctuation, including blanks, are considered separators.

Normally, the rightmost name will be the name of the
originating system. However, it is also permissible to
include an extra entry on the right, which is the name of
the sender. This is for upward compatibility with older
system.

The Path line is not used for replies, and should not be
taken as a mailing address. It is intended to show the
route the message travelled to reach the local site.
There are several uses for this information. One is to
monitor USENET routing for performance reasons. Another
is to establish a path to reach new sites. Perhaps the
most important is to cut down on redundant USENET traffic
by failing to forward a message to a site that is known to
have already received it. In particular, when site A
sends an article to site B, the Path line includes "A",
so that site B will not immediately send the article back
to site A. The site name each site uses to identify
itself should be the same as the name by which its
neighbors know it, in order to make this optimization
possible.

A site adds its own name to the front of a path when it
receives a message from another site. Thus, if a message
with path A!X!Y!Z is passed from site A to site B, B will
add its own name to the path when it receives the message
from A, e.g., B!A!X!Y!Z. If B then passes the message on
to C, the message sent to C will contain the path
B!A!X!Y!Z, and when C receives it, C will change it to
C!B!A!X!Y!Z.

Special upward compatibility note: Since the From, Sender,
and Reply-To lines are in internet format, and since many
USENET sites do not yet have mailers capable of
understanding internet format, it would break the reply
capability to completely sever the connection between the
Path header and the reply function. Thus, sites are
required to continue to keep the Path line in a working
reply format as much as possible, until January 1, 1984.
It is recognized that the path is not always a valid reply
string in older implementations, and no requirement to fix
this problem is placed on implementations. However, the
existing convention of placing the site name and an "!"
at the front of the path, and of starting the path with
the site name, an "!", and the user name, should be
maintained at least until 1984.